1. Field of the Invention
The present invention relates to a photoelectric sensor and, more particularly, to an arrangement in which mutual interference between photoelectric sensors is prevented.
2. Description of the Prior Art
As this kind of photoelectric sensor, a multiple-axis photoelectric sensor disclosed in Japanese Patent Publication No. 2911369 is known. In this photoelectric sensor, a light-projecting device having a plurality of light-projecting elements and a light-receiving device having a plurality of light-receiving elements are opposed to each other and each light-projecting element and the corresponding light-receiving element form an optical axis. A light-projecting scanning operation is repeatedly performed in such a manner that the light-projecting elements provided in the light-projecting device successively project light according to predetermined light-projecting timing. A state in which light from each light-projecting element is blocked at the optical axis is detected on the basis of a received light signal from the light-receiving element which forms the optical axis in association with the light-projecting element. Through the blocked state of light thereby detected, entering of an object into the detection area is sensed.
A plurality of multiple-axis photoelectric sensors may be disposed to detect entering of an object into a wider area. For example, multiple-axis photoelectric sensors may be placed close to each other as shown in FIG. 20. In such a case, when a blocked state at one of the optical axes of the multiple-axis photoelectric sensor 111 in the upper position, for example, is detected, there is a possibility of light projected from the multiple-axis photoelectric sensor 112 in the lower position entering as interfering light into the light-receiving element of the upper multiple-axis photoelectric sensor 111. In such a situation, an error in operation may occur such that when light from the upper multiple-axis photoelectric sensor 111 is blocked at the optical axis, interfering light entering from the lower multiple-axis photoelectric sensor 112 causes failure to detect the blocked state at the optical axis.
To avoid such mutual interference, it is necessary to control the light-projecting scanning operation so that the periods of projection from the multiple-axis photoelectric sensors placed close to each other do not overlap each other. To do so, a sync method is ordinarily used in which one of the two multiple-axis photoelectric sensors 111 and 62 is set as a master station while the other is set as a slave station, a sync signal is transmitted from the master station to the slave station, and the slave station performs the scanning operation with a phase difference from the master station. If this method is used, the multiple-axis photoelectric sensor 112 does not perform detection of a received light signal when one of the light-projecting elements of the other multiple-axis photoelectric sensor 111 is being lighted. Thus, this method has the advantage of preventing the above-described mutual interference.
However, the above-described sync method has a problem in that a troublesome wiring operation is required and the number of operation steps accompanying installation of the devices is large.
Also, this kind of multiple-axis photoelectric sensor is ordinarily arranged to perform an operation for detection as to whether a received light signal is generated from each of light-receiving element at a time when none of light-projecting element is lighted to determine whether interfering light exists. If a synchronization error occurs accidentally in the sync system to cause a coincidence between lighting and timing of detection of interfering light, interfering light is periodically detected since the light-projecting scanning operation is being performed essentially in constant cycles, resulting in determination that a sensor abnormality has occurred.
Also in the case where a plurality of photoelectric sensors each formed of a pair of light-projecting and light-receiving elements are disposed adjacent to each other, interfering light from one of the photoelectric sensors may enter another of the photoelectric sensors to cause an error in operation. To prevent such mutual interference, a method has been proposed in which the photoelectric sensors are connected by a sync line as in the above-described multiple-axis photoelectric sensor, and in which light-projecting timing is controlled so that the photoelectric sensors perform projection and receiving of light at different times. However, the connection of the sync line is also troublesome and there is a need to provide a wiring space. Therefore, it is desirable for ordinary users to avoid use of the sync line.
A method for preventing mutual interference without using a sync line is disclosed in Japanese Patent Laid-Open No. 57-136179. According to this method, the influence of interfering light is avoided by monitoring, at one of a plurality of photoelectric sensors, the entrance of light from another of the photoelectric sensors immediately before the light-projecting time, determining that interfering light from another of the photoelectric sensors is received when the interfering light is received, and shifting the light-projecting time to a later time (away from the time at which interfering light is received).
Each of the above-described methods enables prevention of mutual interference in the case where there are two photoelectric sensors interfering with each other. However, in the case where there are three photoelectric sensors interfering with each other, the method has a drawback described below. That is, the light-projecting period of each photoelectric sensor is determined with reference to a clock oscillated by internal oscillation means. This oscillation means is arranged to output an oscillated clock with a predetermined oscillation period, but has a slight error in the period for each photoelectric sensor. Therefore, in the case where the interval between the time at which light is to be projected from one of the photoelectric sensors and the time at which interfering light comes from another of the photoelectric sensors before the projection of light is gradually reduced by delay of the time of arrival of interfering light, and where the time of projection from the photoelectric sensor from which light is about to be projected is shifted to a later time, light projected from this photoelectric sensor may be superimposed on interfering light subsequently projected from another of the photoelectric sensors. In such a case, mutual interference cannot be prevented.